The present invention pertains to an abrasive article for grinding and polishing glass and a method of using the same.
Glass articles are extensively found in homes, offices, and factories in the form of lenses, prisms, mirrors, CRT screens, and other items. Many of these glass surfaces are used with optical components which require that the surface be optically clear and have no visible defects and/or imperfections. If present, defects, imperfections, and even minute scratches may inhibit the optical clarity of the glass article. In some instances, these defects, imperfections, and/or minute scratches may inhibit the ability to accurately see through the glass. Glass surfaces used with optical components must be essentially free of any defect, imperfection, and/or scratch.
Many glass surfaces are curved or contain a radius associated therewith. These radii and curves are generally generated in the glass forming process. However, as a result of the glass forming process, defects such as mold lines, rough surfaces, small points, and other small imperfections, may be present on the outer surface of the glass. These defects and/or imperfections, however small, tend to affect the optical clarity of the glass. Abrasive finishing processes have been widely used to remove such imperfections and/or defects. The abrasive finishing typically falls within three main processes: grinding, fining, and polishing.
Grinding steps perfect the desired curve or radius and removes any casting defects by rough grinding the glass surface with an abrasive tool. Typically this abrasive tool contains superabrasive particles such as a diamond, tungsten carbide, or cubic boron nitride. However, the abrasive tool in this rough grinding process will impart coarse scratches into the glass surface such that resulting glass surface is neither precise enough nor smooth enough to directly polish to an optically clear state. The objective of the grinding process is to remove large amounts of glass quickly and fairly accurately while leaving as fine of a scratch pattern as feasible. These scratches are then typically removed by further steps commonly known as xe2x80x9cfiningxe2x80x9d and xe2x80x9cpolishingxe2x80x9d.
Glass finishing is typically done with a loose abrasive slurry. The loose abrasive slurry comprises a plurality of abrasive particles dispersed in a liquid medium such as water. The most common abrasive particles used for loose slurries are pumice, silicon carbide, aluminum oxide, and the like. The loose abrasive slurry may optionally contain other additives such as dispersants, lubricants, defoamers, and the like. In most instances, the loose abrasive slurry is pumped between the glass surface that is being finished and a lap pad, such that the loose abrasive slurry is present between the glass surface and the lap pad. The lap pad may be made from any material such as rubber, foam, polymeric material, metal, steel, and the like. Typically, both the glass workpiece and the lap pad will rotate relative to each other. This grinding process typically comprises one or more steps, with each step generating a progressively finer surface finish on the glass.
The roughness of a surface is typically due to scratches or a scratch pattern, which may or may not be visible to the naked eye. A scratch pattern may be defined as a series of peaks and valleys along the surface. Rtm and Ra are common measures of roughness used in the abrasives industry, however, the exact measuring procedure may vary with the type of equipment utilized in surface roughness evaluation.
Ra is defined as an average roughness height value of an arithmetic average of the departures of the surface roughness profile from a mean line on the surface. Measurements are taken at points both above and below the mean line on the surface within an assessment length set by the Rank Taylor Hobson instrument. Ra and Rtm (defined below) are measured with a profilometer probe, which is a 5 micrometer radius diamond tipped stylus and the results are recorded in micrometers (xcexcm). These departure measurements are totaled and then divided by the number of measurements to arrive at an average value. Generally, the lower the Ra value, the smoother the finish.
Rt is defined as the maximum peak-to-valley height. Rtm is the average, measured over five consecutive assessment lengths, of the maximum peak-to-valley height in each assessment length. In general, the lower the Rtm value, the smoother the finish. A slight variation in the Ra and Rtm values may, but not necessarily, occur when the measurement on the same finished glass surface is performed on different brands of commercially available profilometers.
The final step of the overall finishing process is the polishing step which generates the smoother, optically clear surface on the glass article. In most instances, this polishing step is done with a loose abrasive slurry, since the loose slurry typically generates an optically clear surface that is essentially free of any defects, imperfections, and/or minute scratches. Typically, the loose abrasive slurry comprises ceria abrasive particles dispersed in water.
Although loose abrasive slurries are widely utilized in the fining and polishing steps to provide an optically clear surface finish on glass articles, loose abrasive slurries have many disadvantages associated with them. These disadvantages include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive particles and to assure a uniform concentration of abrasive particles at the polishing interface, and the need for additional equipment to prepare, handle, and dispose of or recover and recycle the loose abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability which requires additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the loose abrasive slurry eventually show undesirable wear. Further, the steps which use the slurry are usually very untidy because the loose abrasive slurry, which is a viscous liquid, splatters easily and is difficult to contain.
Understandably, attempts have been made to replace the loose abrasive slurry finishing steps with lapping, coated, or fixed abrasive products. In general, a lapping abrasive comprises a backing having an abrasive coating comprising a plurality of abrasive particles dispersed in a binder. For example, U.S. Pat. Nos. 4,255,164; 4,576,612; 4,733,502; and European Patent Application No. 650,803 disclose various fixed abrasive articles and polishing processes. Other references that disclose fixed abrasive articles include U.S. Pat. Nos. 4,644,703; 4,773,920; and 5,014,468.
However, fixed abrasives have not completely replaced loose abrasive slurries. In some instances the fixed abrasives do not provide a surface, which is optically clear, and essentially free of defects, imperfections, and/or minute scratches. In other instances, the fixed abrasives require a longer time to polish the glass article, thereby making it more cost effective to use a loose abrasive slurry. Similarly in some instances, the life of a fixed abrasive is not sufficiently long to justify the higher cost associated with the fixed abrasive in comparison to loose abrasive slurries. Thus, in some instances, fixed abrasives are not as economically desirable as loose abrasive slurries.
What is desired by the glass industry is an abrasive article that does not exhibit the disadvantages associated with a loose abrasive slurry, but that is able to effectively and economically grind a glass surface in a reasonable time by providing fast stock removal over a short period of time.
One aspect of the invention is directed to abrasive articles for grinding and polishing glass workpieces. The abrasive article for grinding glass includes a backing and at least one three-dimensional abrasive coating comprising diamond particles and a metal salt dispersed in an organic binder, the coating integrally bonded to the backing.
In another aspect, the abrasive article for grinding glass includes a backing and at least one three-dimensional abrasive coating, comprising abrasive agglomerates comprising diamond particles dispersed within a permanent binder, preferably a glass binder, the agglomerates being dispersed within an organic binder integrally bonded to the backing.
In another aspect, the invention provides an abrasive article for polishing glass workpieces comprising a backing comprising fibers and at least one three-dimensional abrasive coating integrally bonded to the backing, the coating comprising ceria particles, an organic binder, and a metal salt. The abrasive articles of the invention for polishing provide an optimum rate of breakdown of the abrasive composites and improved adhesion between the abrasive composites and the backing. The abrasive article for polishing glass preferably further comprises an additive for improving the polishing rate. Preferred additives include graphite and molybdenum disulfide.
It is preferred that the at least one three-dimensional abrasive coating includes a plurality of abrasive composites. The plurality of abrasive composites may be precisely shaped composites, irregularly shaped composites, or precisely shaped composites including a cylinder or any other post-shape having a flat top.
In one preferred abrasive article, the abrasive composites comprise epoxy binder and are integrally molded urethane backing. Acrylates and urethane acrylates may also be usable as a preferred material.
For grinding glass, it is preferred that the abrasive particles comprise diamond abrasive particles or diamond agglomerate particles. Optionally, the diamond particles may be blended with other non-diamond hard abrasive particles, soft inorganic abrasive particles, and mixtures thereof. For polishing glass, it is preferred that the abrasive particles comprise ceria particles.
It is preferred that the diamond abrasive particles are present in the abrasive composite in a weight percent between about 0.1% to 10%, preferably between about 2% to 4%. The preferred amount of ceria abrasive particles is up to 85% by weight.
In one embodiment of the invention, the abrasive article for grinding glass is capable of removing between 200 and 400 micrometers stock on a glass test blank to a final Ra of about 1.1 xcexcm or less using an RPP procedure having a grinding time interval of between 10 and 15 seconds, generally about 12 seconds.
In another embodiment of the invention, the abrasive article for grinding glass is capable of removing 200 micrometers stock on a glass test blank to a final Ra of about 0.80 xcexcm or less using an RPP procedure having a grinding time interval of between 10 and 15 seconds, generally about 12 seconds.
In yet another embodiment of the invention for grinding glass, the abrasive article containing agglomerates containing diamond particles is capable of removing 100 micrometers stock on a glass test blank to a final Ra of about 0.7 xcexcm using an RPP procedure having a grinding time interval of between 10 and 15 seconds, generally about 12 seconds.
In yet another embodiment of the invention for polishing glass, the abrasive article containing ceria is capable of reducing surface roughness of Ra 0.07 xcexcm to a surface roughness of about 0.009 xcexcm using a polish time interval of about 45 seconds or less, preferably in about 30 seconds or less, more preferably about 20 seconds or less and even more preferably, about 15 seconds.
The RPP Test Procedure
The xe2x80x9cRPPxe2x80x9d procedure utilizes a xe2x80x9cBuehler Ecomet 4xe2x80x9d variable speed grinder-polisher on which is mounted a xe2x80x9cBuehler Ecomet 2xe2x80x9d power head, both of which are commercially available from Buehler Industries, Ltd. of Lake Bluff, Ill. The test is typically performed using the following conditions: motor speed set at 500 rpm with a force 60 lbs. (267 N), which provides an interface pressure of about 25.5 psi (about 180 kPa) over the surface area of the glass test blank. The interface pressure may be increased or decreased for testing under varied conditions.
Three flat circular glass test blanks are provided which have a 2.54 cm (1 inch) diameter and a thickness of approximately 1.0 cm, commercially available under the trade designation xe2x80x9cCORNING #9061xe2x80x9d, commercially available from Corning Incorporated, Corning, N.Y. The glass material is placed into the power head of the grinder-polisher. The 12-inch (30.5 cm) aluminum platform of the grinder-polisher rotates counter clockwise while the power head, into which the glass test blank is secured, rotates clockwise at 35 rpm.
An abrasive article to be tested is die cut to a 20.3 cm (8 inch) diameter circle and is adhered with a pressure sensitive adhesive directly onto an extruded slab stock foam urethane backing pad which has a Shore A hardness of about 65 durometer. The urethane backing pad is attached to an extruded slab open cell, soft foam pad having a thickness of about 30 mm. This pad assembly is placed on the aluminum platform of the grinder/polisher. Tap water is sprayed onto the abrasive article at a flow rate of approximately 3 liters/minute to provide lubrication between the surface of the abrasive article and the glass test blank.
An initial surface finish on the glass test blank is evaluated with a diamond stylus profilometer, commercially available under the trade designation xe2x80x9cSURTRONIC 3xe2x80x9d, commercially available from Taylor Hobson, Leicester, England. An initial thickness and weight of the glass test blank is also recorded.
The glass test blank is ground using the grinder described above. The grinding time interval of the grinder is set at 10 seconds. However, real time contact between the abrasive article and the glass test blank surface may be greater than the set time because the grinder will not begin timing until the abrasive article is stabilized on the glass test blank surface. That is, there may be some bouncing or skipping of the abrasive article on the glass surface and the grinder begins timing at the point when contact between the abrasive article and the glass surface is substantially constant. Thus, real time grinding interval, that is the contact between the abrasive article and the glass surface, is about 12 seconds. After grinding, final surface finish and a final weight or thickness are each recorded.
It will be understood that the actual time (rate) necessary to grind an actual glass workpiece to the desired specification will vary depending upon a number of factors, such as the polishing apparatus used, the backing pad under the abrasive article, the speed of the abrasive rotation, the size of the surface area to be polished, the contact pressure, the abrasive particle size, the amount of glass to be removed, and the initial condition of the surface to be ground, etc. The RPP procedure above simply provides a baseline performance characteristic that may be used to compare the article and the method according to the invention with conventional glass grinding techniques.